1. Technical Field of the Invention
The present invention relates generally to test and measurement systems, and specifically to measurement of the jitter in electrical and optical data streams.
2. Description of Related Art
Jitter is the deviation of the actual zero-crossing time of a signal from the ideal zero-crossing time. The signal zero-crossing time refers to the time that the amplitude of a data signal crosses a decision threshold (e.g., one-half amplitude) of a signal transition (e.g., a rise or fall of the signal). Measuring the jitter in serial data communications systems ensures that the jitter (if any) present in the system does not cause errors. There are a number of techniques for measuring jitter at lower frequencies. Examples of devices that have been used to measure the jitter in data streams include real-time digital oscilloscopes, phase-detector-based jitter measurement devices that include clock recovery circuits to convert the data stream into a periodic clock signal and time interval measurement devices that use a trigger circuit to measure the timing of a signal zero-crossing.
However, as signaling rates of serial communications signals increase, e.g. up to and above 40 Gb/s, measuring the jitter becomes problematic. For example, real-time digital oscilloscopes have inadequate bandwidth (currently about 4 GHz) to faithfully digitize 40 Gb/s waveforms. In addition, clock recovery circuits at 40 Gb/s suffer from severe tradeoffs between jitter transfer bandwidth (i.e., the limit on the low-pass characteristic of the clock recovery circuit) and jitter noise floor (i.e., the limit on the suppression of unintentional jitter not related to the jitter on the original signal). Furthermore, at 40 Gb/s, the limited bandwidth, phase distortion and intrinsic jitter contribution of current trigger circuit technology in time interval measurement devices would significantly distort the measurement result.
High-speed electronic sampling oscilloscopes offer one solution to the problem of measuring the jitter in high data rate signals. High-speed electronic sampling oscilloscopes use a single trigger circuit that triggers a sampling strobe to generate a pulse upon the detection of a zero-crossing in a trigger signal at the input of the trigger circuit. Each output pulse drives a sampler that measures the amplitude of the signal-under-test (SUT) at the ideal zero-crossing time. However, high-speed electronic sampling oscilloscopes are limited in their ability to measure the jitter.
For example, in high-speed electronic sampling oscilloscopes, the sample rate (usually <1 MHz) is much lower than the signal rate, and therefore no more than a single sample is taken during any one particular signal zero-crossing. If the signal data is random or a pattern trigger is not available, the direction of the signal zero-crossing (up or down) is unknown and the time deviation is ambiguous. In addition, the sampling oscilloscope strobe is usually provided by either an asynchronous trigger or a frequency-synthesized periodic oscillator, either of which contribute at least 0.6 ps of rms jitter to the measurement. The additional 0.6 ps of rms jitter produces a significant impact on a 40 Gb/s measurement.
In addition, the low sampling rate in high speed electronic sampling oscilloscopes makes it impossible to analyze the time interval jitter between two nearby or adjacent zero-crossings. The time interval jitter is the deviation in the actual time interval between two nearby or adjacent measured zero-crossings from the ideal time interval between the two nearby or adjacent zero-crossings. Measuring the time interval jitter enables a frequency-domain or auto-correlation jitter analysis, such as the jitter analysis algorithm described in PCT International Application WO 99/39216 to Wilstrup et al. (hereinafter referred to as the Wavecrest technique), which is hereby incorporated by reference.
In Wilstrup et al., two trigger circuits are used to measure the time interval between two nearby or adjacent zero-crossings. Each trigger circuit is set to generate an output pulse upon the detection of a different zero-crossing in the signal. Circuitry connected to the two trigger circuits receives the two pulses and compares the timing of the two pulses to measure the time interval between the two pulses. The measured time interval is compared to the ideal time interval to determine the time interval jitter.
However, the time interval jitter measurement includes not only the jitter present in the signal, but also the jitter inherent within and between the two trigger circuits. At low frequencies, the inherent jitter within the two trigger circuits does not significantly effect the time interval jitter measurement. However, as signaling rates of serial communications signals increase, e.g. up to and above 40 Gb/s, the limited bandwidth, phase distortion and intrinsic jitter contribution of the two trigger circuits in Wilstrup et al. significantly distorts the measurement result. Thus, the jitter measurement system described in Wilstrup et al. does not provide an accurate time interval jitter analysis. Therefore, what is needed is a high data rate sampling apparatus for use in jitter analysis that is capable of providing plural samples within a single signal transition and reduces the effects of the inherent jitter present in the trigger circuit on the jitter measurement.